The present invention relates generally to wireless local area networks, and more particularly to synchronization of wireless local area network access points.
Popular communications services, such as access to the global Internet, e-mail, and file downloads, are provided via connections to packet data networks. To date, user devices, such as personal computers, have commonly connected to a packet data network via a wired infrastructure. For example, a patch cable connects the Ethernet port on a personal computer to an Ethernet wall jack, which is connected by infrastructure cabling running through the walls of a building to network equipment such as a switch or router. There are disadvantages to a wired infrastructure. From a network perspective, providing packet data services to homes and commercial buildings requires installation of infrastructure cabling. From a user perspective, access to the network is limited to availability of a wall jack, and mobility is limited by the length of the patch cable.
Wireless local area networks (WLANs) provide advantages both for network provisioning and for customer services. For a network provider, a WLAN reduces required runs of infrastructure cabling. For a network user, a WLAN provides ready access for mobile devices such as laptop computers and personal digital assistants. WLANs were first deployed in commercial installations. As the price of WLAN network equipment has dropped, and as residential broadband services have become widely deployed, however, WLANs in homes are becoming commonplace. In a typical installation, a wireless router connects to a modem which connects to an Internet service provider (ISP) via a broadband access line such as digital subscriber line (DSL), cable, or fiber optics. A laptop, for example, outfitted with a wireless modem card, then communicates with the wireless router over a radiofrequency (RF) link. The wireless router provides access to the ISP for multiple laptops within the RF coverage area of the wireless router.
WLANs may be configured via various network schemes. Some are proprietary, and some follow industry standards. At present, many widely deployed WLANs follow the IEEE 802.11 standards. WLANs based on these standards are popularly referred to as WiFi. In examples discussed below, WLANs refer to networks based on the IEEE 802.11 standards. Embodiments of the invention, however, are applicable to other WLANs.
WLANs may be further classified by architecture. In a mobile ad-hoc network, wireless devices, such as laptops outfitted with wireless modems, communicate directly with each other in a peer-to-peer mode. In an infrastructure network, wireless devices communicate via an RF connection to an access point. A home WLAN typically will be served by a single access point, such as the wireless router. Wireless devices within the RF coverage area of the wireless router connect to the broadband service via the single access point. Wireless devices may also communicate with each other via the single access point.
To provide RF coverage over a wider area, such as in an airport, multiple access points are often required. The RF coverage areas of multiple access points may overlap. In some instances, the overlap is intentional to provide seamless coverage. In other instances, the overlap is unintentional since the boundaries of RF coverage areas are not sharply defined. RF coverage areas may also overlap if more than one network is operating in the same location. For example, in a commercial environment, competing network providers may be offering services in the same, or an adjacent, location. In a residential environment, such as an apartment complex or a neighborhood in which houses are close together, WLANs independently operated by neighbors may overlap in RF coverage area.
In a widely deployed WLAN protocol, devices communicate with each other over a common channel on a contention basis. That is, each device attempts to seize the channel. At a given instance, if there are multiple contending devices, the device which actually seizes the channel is governed by a carrier sense multiple access/collision avoidance (CSMA/CA) protocol and a random backoff mechanism. When a device wants to transmit, it first senses the medium (RF channel) to determine whether the medium is busy. The medium is busy if there is already data traffic on the RF channel. If the medium is not busy, the device transmits after a delay period. If the medium is determined to be busy, the device defers until the end of the current transmission. After deferral, or prior to attempting to transmit again immediately after a successful transmission, the device selects a random backoff interval and decrements a backoff interval counter while the medium is idle.
In addition to the above CSMA/CA channel access mechanism, WLANs may support another channel access mechanism called scheduled access mode. Under this mode, devices are individually polled by a channel access coordinator. For example, in a WLAN controlled by a single access point, the access point may serve as a channel access coordinator to coordinate the devices in the network. Under scheduled access mode, devices do not need to execute the carrier sensing and backoff mechanisms. At a given instance, only the one device being polled is allowed to transmit. Also, due to the deterministic nature of scheduled access, service quality, in some instances, may be guaranteed by the channel access coordinator's polling schedule. The same WLAN system may support both CSMA/CA and scheduled access modes. In such a WLAN system, time periods in which the system operates in CSMA/CA mode, known as contention periods (CP), alternate with time periods in which the system operates in scheduled access mode, known as contention-free periods (CFPs).
For a specific WLAN, devices communicate with each other over a common channel. If a second network, whose RF coverage area overlaps the RF coverage area of the first network, operates on the same channel, then co-channel interference may occur. Co-channel interference may occur, for example, if there is no time synchronization and network coordination between the access points in the two networks. Transmissions from devices and access points in the second network may interfere with transmissions from devices and access points in the first network. That is, collision between data frames may occur, and the data throughput of the first network may effectively be reduced. The data throughput of the second network may similarly be reduced because of interfering transmissions from devices and access points in the first network.
As discussed in more detail below, transmission intervals are partitioned into well-defined beacon intervals, which are delimited by beacon frames. A beacon interval starts with a first beacon frame, which contains timing information and other network control parameters. The beacon interval ends with a second beacon frame, which also marks the beginning of the next beacon interval. In some network configurations, the beacon frame is used to synchronize the clocks on the devices to the clock on the access point. The sequence of beacon intervals is then synchronized throughout the WLAN. In IEEE 802.11 systems that enable both CSMA/CA and scheduled access modes, the time period that immediately follows the beacon frame is dedicated for CFP, and the rest of the beacon interval is dedicated for CP. As a result, there are well-defined CFPs during which the access point does not need to contend with traffic from the devices. During the CFPs, WLAN devices may also transmit high-priority messages and messages with high quality of service (QoS) requirements.
The above synchronization mechanism may work well if a single access point synchronizes all the devices operating on the common channel. If two independent access points have overlapping RF coverage areas and operate over the same channel, however, devices located in the overlap area may be subjected to co-channel interference when they operate during CFPs. This may occur, for example, if a CFP in the first network overlaps a CFP in the second network. If a device in the first network and a device in the second network simultaneously transmit in a CFP mode, there may be a high probability of collision between data frames.
In general, there is no synchronization between the clocks of different access points. Also, in general, there is no coordination between the operations of different access points. The IEEE 802.11 standards do not explicitly address synchronization and coordination of different access points. Various options are available. For example, consider two access points. If the first access point receives the beacon messages (carried in the beacon frames) from the second access point, then the first access point may determine the CFP configuration of the second access point. The first access point may then adjust its own CFP configuration to reduce, minimize, or eliminate co-channel interference. Similarly, if the second access point receives the beacon messages from the first access point, the second access point may adjust its own CFP configuration accordingly.
In a more general case, however, an access point does not necessarily receive beacon messages from other access points, and a different method for synchronization is required. Herein, wireless network elements refer to wireless access points and wireless devices. In general, a wireless network element has a system clock that provides timing for control and communication processes running in the wireless network element, and a separate radio clock that provides timing for the RF transmitter and receiver. The beacon intervals and their corresponding CFPs are referenced with respect to the radio clock. In general, the system clock and the radio clock are not well synchronized. System clocks of different network elements (wireless or wired) may be synchronized by mechanisms such as network time protocol (NTP), in which the system clocks are slaved to a master clock over a network. These mechanisms, however, do not synchronize their radio clocks. Additionally, heavy network traffic may degrade network synchronization. Network design and operations may also make translating system clock synchronization into radio clock synchronization difficult. What is needed is method and apparatus for providing stable, high resolution synchronization of radio clocks of multiple access points.